Network Working Group J. Rosenberg
Request for Comments: 3858 dynamicsoft
Category: Standards Track August 2004
An Extensible Markup Language (XML) Based Format for Watcher
Information
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
Watchers are defined as entities that request (i.e., subscribe to)
information about a resource. There is fairly complex state
associated with these subscriptions. The union of the state for all
subscriptions to a particular resource is called the watcher
information for that resource. This state is dynamic, changing as
subscribers come and go. As a result, it is possible, and indeed
useful, to subscribe to the watcher information for a particular
resource. In order to enable this, a format is needed to describe
the state of watchers on a resource. This specification describes an
Extensible Markup Language (XML) document format for such state.
Table of Contents
1. Introduction ................................................ 2
2. Terminology ............................................... 2
3. Structure of Watcher Information ........................... 2
4. Computing Watcher Lists from the Document .................. 5
5. Example .................................................... 6
6. XML Schema ................................................. 6
7. Security Considerations .................................... 8
8. IANA Considerations ........................................ 9
8.1. application/watcherinfo+xml MIME Registration ......... 9
8.2. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:watcherinfo .................... 10
9. Normative References ....................................... 10
10. Informative References ..................................... 11
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11. Acknowledgements ........................................... 11
12. Contributors ............................................... 12
13. Author's Address ........................................... 13
14. Full Copyright Statement ................................... 14
1. Introduction
Watchers are defined as entities that request (i.e., subscribe to)
information about a resource, using the SIP event framework, RFC 3265
[1]. There is fairly complex state associated with these
subscriptions. This state includes the identity of the subscriber,
the state of the subscription, and so on. The union of the state for
all subscriptions to a particular resource is called the watcher
information for that resource. This state is dynamic, changing as
subscribers come and go. As a result, it is possible, and indeed
useful, to subscribe to the watcher information for a particular
resource. An important application of this is the ability for a user
to find out the set of subscribers to their presentity [11]. This
would allow the user to provide an authorization decision for the
subscription.
To support subscriptions to watcher information, two components are
needed. The first is the definition of a SIP event template-package
for watcher information. The other is the definition of a data
format to represent watcher information. The former is specified in
[2], and the latter is specified here.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[3] and indicate requirement levels for compliant implementations.
This document also uses the terms subscriber, watcher, subscription,
notification, watcherinfo subscription, watcherinfo subscriber, and
watcherinfo notification with the meanings described in [2].
3. Structure of Watcher Information
Watcher information is an XML document [4] that MUST be well-formed
and SHOULD be valid. Watcher information documents MUST be based on
XML 1.0 and MUST be encoded using UTF-8. This specification makes
use of XML namespaces for identifying watcherinfo documents and
document fragments. The namespace URI for elements defined by this
specification is a URN [5], using the namespace identifier 'ietf'
defined by [6] and extended by [7]. This URN is:
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RFC 3858 Watcher Info August 2004
urn:ietf:params:xml:ns:watcherinfo
A watcher information document begins with the root element tag
"watcherinfo". It consists of any number of "watcher-list" sub-
elements, each of which is a list of watchers for a particular
resource. Other elements from different namespaces MAY be present
for the purposes of extensibility; elements or attributes from
unknown namespaces MUST be ignored. There are two attributes
associated with the "watcherinfo" element, both of which MUST be
present:
version: This attribute allows the recipient of watcherinfo documents
to properly order them. Versions start at 0, and increment by one
for each new document sent to a watcherinfo subscriber. Versions
are scoped within a watcherinfo subscription. Versions MUST be
representable using a 32 bit integer. However, versions do not
wrap.
state: This attribute indicates whether the document contains
the full watcherinfo state, or whether it contains only
information on those watchers which have changed since the
previous document (partial).
Each "watcher-list" element contains a list of "watcher" elements,
each of which describes a watcher on a particular resource. Other
elements from different namespaces MAY be present for the purposes of
extensibility; elements or attributes from unknown namespaces MUST be
ignored. There are two attributes associated with the "watcher-list"
element, both of which MUST be present:
resource: This attribute contains a URI for the resource being
watched by that list of watchers. It is mandatory.
package: This attribute contains a token indicating the event
package for which watcher information on that resource is being
provided. It is mandatory.
The "watcher" element describes a watcher in the watcher list. The
value of the "watcher" element is a URI for the watcher. This URI
SHOULD be the authenticated identity of the watcher as determined by
the server processing the subscription. As such, this URI will
usually be an address-of-record (for example, sip:joe@example.com) as
opposed to a device address (for example, sip:joe@192.0.2.3). There
are three mandatory attributes which MUST be present:
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id: A unique identifier for the subscription described by the
watcher element. The id MUST be representable using the grammar
for token as specified by RFC 3261 [8]. It MUST be unique across
all other watchers reported in documents sent in notifications for
a particular watcherinfo subscription.
status: The status of the subscription. The meaning of the
various statuses are defined in the watcher information event
package [2].
event: The event which caused the transition to the current
status. The events are defined in the watcher information event
package [2].
There are also some optional, informative attributes of the watcher
element. These are:
display-name: A textual representation of the name of the
subscriber.
expiration: The amount of time, in seconds from the current
time, that the subscription will expire.
duration-subscribed: The amount of time, expressed in seconds,
between the time the SUBSCRIBE which created the subscription was
received, and the current time.
The xml:lang attribute MAY be used with the "watcher" element to
specify the language of the "display-name".
The number of watchers present for each resource, and the set of
resources listed, depends on the type of data being provided, and to
whom.
For example, consider a presence system using watcher information. In
one scenario, a user, A, subscribes to the presence of another user,
B. A would like to find out about the status of their subscription.
To do so, A subscribes to the watcher information for B's presence.
A does not have authorization to learn the status of all watchers for
B's presence. As a result, the watcher information sent to A will
contain only one watcher - A themself.
In another scenario, a user, B, wishes to learn the set of people who
have subscribed to B's presence. To do this, B subscribes to the
watcher information for B's presence. Here, B is authorized to see
all the watchers of B's presence. As a result, the watcher
information sent to B will contain all watchers of B's presence.
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In the case where an administrator wishes to learn the current status
in the system, the watcher information could contain all watchers for
all resources.
4. Computing Watcher Lists from the Document
Typically, the watcherinfo NOTIFY will only contain information about
those watchers whose state has changed. To construct a coherent view
of the total state of all watchers, a watcherinfo subscriber will
need to combine NOTIFYs received over time. The watcherinfo
subscriber maintains a table for each watcher list it receives
information about. Each watcher list is uniquely identified by the
URI in the "resource" attribute of the "watcher-list" element. Each
table contains a row for each watcher in that watcher list. Each row
is indexed by the unique ID for that watcher. It is conveyed in the
"id" attribute of the "watcher" element. The contents of each row
contain the state of that watcher as conveyed in the "watcher"
element. The tables are also associated with a version number. The
version number MUST be initialized with the value of the "version"
attribute from the "watcherinfo" element in the first document
received. Each time a new document is received, the value of the
local version number, and the "version" attribute in the new
document, are compared. If the value in the new document is one
higher than the local version number, the local version number is
increased by one, and the document is processed. If the value in the
document is more than one higher than the local version number, the
local version number is set to the value in the new document, the
document is processed, and the watcherinfo subscriber SHOULD generate
a refresh request to trigger a full state notification. If the value
in the document is less than the local version, the document is
discarded without processing.
The processing of the watcherinfo document depends on whether it
contains full or partial state. If it contains full state, indicated
by the value of the "state" attribute in the "watcherinfo" element,
the contents of all tables associated with this watcherinfo
subscription are flushed. They are re-populated from the document.
A new table is created for each "watcher-list" element, and a new row
in each table is created for each "watcher" element. If the
watcherinfo contains partial state, as indicated by the value of the
"state" attribute in the "watcherinfo" element, the document is used
to update the existing tables. For each "watcher-list" element, the
watcherinfo subscriber checks to see if a table exists for that list.
This check is done by comparing the URI in the "resource" attribute
of the "watcher-list" element with the URI associated with the table.
If a table doesn't exist for that list, one is created. For each
"watcher" element in the list, the watcherinfo subscriber checks to
see whether a row exists for that watcher. This check is done by
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comparing the ID in the "id" attribute of the "watcher" element with
the ID associated with the row. If the watcher doesn't exist in the
table, a row is added, and its state is set to the information from
that "watcher" element. If the watcher does exist, its state is
updated to be the information from that "watcher" element. If a row
is updated or created, such that its state is now terminated, that
entry MAY be removed from the table at any time.
5. Example
The following is an example of watcher information for a presentity,
who is a professor. There are two watchers, userA and userB.
sip:userA@example.netsip:userB@example.org
6. XML Schema
The following is the schema definition of the watcherinfo document
format: